In conventional grinding mills the seeds or grains to be ground are loaded into a hopper from whose outlet they are extracted by a transport roller and/or a feedscrew for delivery to the nip of two counterrotating main rolls or cylinders between which they are comminuted. On their way to these grinding cylinders the grains move through a narrow passage which is bounded on one side by a curved cover plate whose spacing from a fixed countersurface is controlled--e.g. hydraulically--by a seed-level detector such as a float located in the hopper. Since the reloading of the hopper generally takes place only at intervals, the level of the granular mass therein is subjected to considerable variations and may, in fact, go to zero when the contents of the hopper are depleted. In such a case, the detector should separate the two counterrotating cylinders from each other in order to prevent them from coming into direct grinding contact; at the same time, the detector arrests the transport roller and moves the cover plate onto the countersurface in order to block the passage of grain therethrough until the hopper has been refilled whereupon the cylinders can be restored to their operating position.
In order to let the mill operate as efficiently as possible with changing loads, the detector may also cause a shifting of the cover plate during normal operating so as to vary the width of the passage in a manner generally proportional to the seed level in the hopper. This is meant to maintain a substantially constant density of the mass to be ground.
Such a change of the width of the granular flow reaching the grinding cylinders, however, may bring about significant variations in the granulometry and thus in the chemical and physical characteristics of the product. This reflects on the overall quality of the flour and is therefore objectionable.